1. Field of the Invention
This invention relates to the so-called hot isostatic pressing (hereinafter abbreviated as "HIP" for the sake of brevity) apparatus which is adapted to conduct the sintering or densification of a ceramic or metal powder compact in a gaseous atmosphere of high temperature and high pressure, and more particularly to an HIP apparatus equipped internally with a heat-insulating layer and heater, which HIP apparatus is useful to make up a modular HIP system in combination with a cooling station and optionally an unloading station as well as a carrier apparatus adapted to carry a treated object, the heater and the heat-insulating layer together with one of lids of a pressure vessel along the HIP apparatus and the cooling and unloading stations.
2. Description of the Prior Art
An HIP treatment comprises pressing an object isotropically under elevated temperatures while using an inert gas as a pressure medium, and is a technique which has attracted considerable attention in recent years from various fields as a method excellent for producing a sintered dense body from ceramics powder or metal powder or a mixture thereof, squeezing off residual voids in a superhard alloy, or diffusion-bonding metal materials.
Since such an HIP treatment is carried out in an atmosphere of high temperature and high pressure, it is indispensable to conduct it over a long cycle time for raising the temperature, raising the pressure, lowering the temperature and then lowering the pressure in an HIP apparatus which has a special structure and is costly. Therefore, it has been considered to be an important technical subject to shorten the cycle time and to improve the efficiency of each HIP treatment. With a view toward solving the above subject, many attempts have heretofore been made to improve the utilization efficiency of an HIP apparatus in terms of its working hours by conducting the heating, i.e., the temperature-raising step of an object, which step requires a long time period, in a preheating furnace and effecting only the pressure-raising step of the object plus an optional slight temperature-raising step in the HIP apparatus. As a representative example of such attempts, mention should be made of the invention proposed in British patent specification No. 1,291,459. The apparatus of the above British patent certainly has merit in that the cycle time thereof can be shortened. However, it is accompanied by such drawbacks as that it requires a preheating furnace in addition to an ordinary high-pressure and high-temperature treatment apparatus and thus raises the equipment cost, and an extremely great deal of heat is lost due to radiation of heat because the transportation of each preheated object is carried out in the atmosphere. An additional drawback is that, when loading a preheated object in a pressure cylinder, the inner wall of the pressure cylinder may be overheated underneath the preheated object due to the heat of the preheated object and this overheating of the inner lower wall of the pressure cylinder damage the lower hermetic ring. Accordingly, the present assignee made an improvement to the above apparatus and has already proposed, in Japanese Patent Laid-open No. 124,610/1976, an HIP apparatus which is capable of shortening the cycle time and enjoys a high level of safety without adversely affecting any parts of the apparatus such as the pressure cylinder. The features of the invention proposed in the above Japanese patent specification reside in the fact that, in a high-pressure and high-temperature furnace constructed of a pressure chamber which is formed of a pressure cylinder and upper and lower plugs respectively and hermetically sealing the upper and lower openings of the pressure cylinder, a heat-insulating layer disposed within the pressure chamber and a heater arranged inside the heat-insulating layer and adapted to subject an object placed on the lower plug to a treatment such as sintering or bonding in a gaseous atmosphere of high pressure and high temperature, the lower plug, heater and heat-insulating layer are removably provided as an integral unit from the pressure cylinder, the treatment chamber defined by the heat-insulating layer and lower plug is rendered openable, hermetic sealing means are provided with the treatment chamber, gas flow passages are provided with the lower plug so as to communicate the exterior of the lower plug with the exterior and interior of the treatment chamber, and the gas flow passages are communicated with each other by means of a valve. According to the above-noted invention, it is possible not only to shorten the cycle time owing to the preheating of an object but also to minimize the heat loss of the object when carrying the pre-heated object of a high temperature into the pressure chamber. Since the inner wall of the pressure cylinder is not overheated by the heat of the preheated body and the service life of the pressure cylinder can thus be prolonged, the above-noted HIP apparatus is safe. In addition, the preheating can be carried out in vacuum or in an atmosphere of a specific inert gas. It has become possible to use, for the heater and heat-insulation layer, materials which are susceptible to oxidation at high temperatures.
In the meantime, there was also developed the so-called modular HIP system in which one or more auxiliary stations adapted to preheat or cool an object in an inert gas atmosphere is combined with a single unit of such an HIP apparatus as described above, the HIP apparatus and auxiliary stations are connected by a track laid horizontally therebetween, and the object and heater are, after being rapidly depressurized in the HIP apparatus and while being still at a high temperature, unloaded together with the inert gas atmosphere from the HIP apparatus and then separately cooled in the auxiliary station or the object and heater are preheated before loading them in the HIP apparatus.
The above modular HIP apparatus has shortened the cycle time of HIP treatment to a considerable extent and materialized a highly efficient HIP treatment compared with any HIP apparatus used or proposed by that time, because the cooling, preheating, loading and/or unloading of objects are conducted at auxiliary stations while a still further object is subjected to an HIP treatment in the HIP apparatus. Thus, such is expected to find more commercial utility in the future. However, it has also been found that the above modular HIP apparatus is accompanied by the following drawbacks.
Namely, the heat-insulating layer of an HIP apparatus has such high performance that the inner wall of the pressure vessel is maintained at temperatures below 100.degree.-200.degree. C. even when the atmosphere in the treatment chamber is, for example, at 2000.degree. C. and 2000 atms. When cooling an HIP-treated object in an auxiliary station after the object has been treated in the above modular HIP system, the heat-insulating performance of the heat-insulating layer is now too good and the cooling of the object requires a long period of time.